Recently, there have been remarkable developments in the field of information processing (e.g., a field of personal computer) and the field of wireless communication (e.g., a field of cellular phone). Improvement of signal transmission speed of a board and realization of low transmission loss in a high-frequency region are required in order to improve information processing speed in these fields. Signal-transmission speed increases as the dielectric constant is low. In addition, since waveform distortion becomes smaller as the dielectric constant becomes lower, development of a high-frequency circuit board having a low dielectric constant has been investigated.
Conventionally, ceramics have been used in these applications. However, ceramics have problems in that ceramics are difficult to work and expensive. Therefore, there have been demands for easily workable and inexpensive organic materials as alternative materials for ceramics. For example, as applications of organic materials, there have been proposed an use of a fluorine resin having excellent dielectric property as an electric insulator of a board (hereinafter, referred to as a “PTFE board”) or an use of polyimide having excellent heat resistance as an electric insulation layer of a board (hereinafter, referred to as a “PI board”).
In the PTFE board, the fluorine resin itself has excellent high-frequency property and moisture resistance. However, high-frequency property and moisture resistance of the entire board is low due to influence of a glass fiber cloth and the like used for improving dimensional stability. High-frequency property of the PI board is remarkably inferior to that of the PTFE board. In addition, the PI board has high hygroscopicity, and therefore, the high-frequency property is significantly deteriorated by moisture absorption.
Patent Document 1 (JP Laid-open Patent Publication No. H11-309803) discloses a multilayer laminated plate, a method for producing the same, and a multilayer mounting circuit board.
This Document discloses a multilayer laminated plate comprising a plurality of laminated bodies bonded by thermo-compression bonding, wherein a film produced from a polymer that can form an optically anisotropic melt phase and a support body are laminated in each laminated body. Where two adjacent laminated bodies are disposed in such a state that a support body of one laminated body faces a support body of the other laminated body, the two laminated bodies are bonded with an intermediate sheet in between. The intermediate sheet is made of a film produced from a polymer that can form an optically anisotropic melt phase. The film of the laminated body and the intermediate sheet have the same chemical composition, whereas adjacent film and the intermediate sheet are provided with different heat resistance.
Patent Document 2 (JP Laid-open Patent Publication No. 2000-263577) discloses a method for producing a metal foil laminated plate and the metal foil laminated plate.
This document discloses a method for producing a metal foil laminated plate, overlapping constituent materials composed of a film made of a thermoplastic polymer that can form an optically anisotropic melt phase (hereinafter, this is referred to as a “thermoplastic liquid crystal polymer film”) and a metal foil, sandwiching the film and the metal foil by two flat metal plates to form a unit laminate, stacking a plurality of sets of the unit laminate to form a stacked body, disposing the stacked body between opposed press heating plates, and forming the metal foil laminated plate by a process including hot-pressing the stacked body, wherein the process includes (1) a first step (preliminary heating step) of heating the stacked body to a preliminary heating temperature that is at most 30° C. lower than a melting point of thermoplastic liquid crystal polymer film without pressurizing the stacked body, (2) a second step (temperature elevation step) of heating the stacked body from the preliminary heating temperature to a laminating temperature selected from a range between a lower limit being 5° C. lower than the melting point of the thermoplastic liquid crystal polymer film and an upper limit being 5° C. higher than the melting point while pressurizing the stacked boy with a pressure maintained at 2 kg/cm2 or less, (3) a third step (pressurizing step) of pressurizing the stacked body with a pressure selected from the range of 20 kg/cm2 to 50 kg/cm2 at the laminating temperature, and (4) a fourth step (cooling step) of cooling the stacked body to a cooling temperature at least 30° C. lower than the melting point of the thermoplastic liquid crystal polymer film while maintaining the pressure of the pressurizing step, wherein the second step to the fourth step are performed within 30 minutes, and the first step to the fourth step are performed under a vacuum condition of 30 torr or less, and a metal foil laminated film is unloaded in a fifth step (ejection step) after releasing the compression and vacuum condition.